This invention relates to a molten silicon infiltration formed silicon carbide composite and a method for forming the composite, and more specifically to a protective coating having a reactive interlayer on a reinforcement phase in the composite.
High temperature fiber reinforced composites have great potential for use in aircraft and gas turbines due to the high strength to weight ratio of such materials. Composites of carbon fiber reinforced carbon matrices have been used in aircraft construction, but poor oxidation resistance has limited use to low temperature applications of 1000.degree. C. or less. High strength carbon fibers have been infiltrated with molten silicon to provide a silicon matrix for protecting the carbon fiber reinforcements. However, the silicon infiltration converts the carbon fiber reinforcements into relatively weak, irregular columns of silicon carbide crystals resulting in composites with low toughness and relatively modest strength.
As an alternative approach, attempts have been made to incorporate silicon carbide fibers in a silicon matrix by the process of silicon infiltration. Unfortunately, silicon carbide has a limited solubility in molten silicon, and leads to transport and recrystallization of silicon carbide causing the silicon carbide fibers to loose substantial strength. Also, silicon carbide forms a strong bond with silicon so the fiber bonds to the matrix resulting in brittle fracture of the composite. In ceramic composites, a relatively weak bond at the fiber-matrix interface is preferred in order to achieve improved fracture toughness. Toughness is improved in fiber reinforced ceramic composites when the fiber reinforcement does not chemically bond with the surrounding matrix, so that force applied to the matrix is transferred from the matrix to the fiber substantially by friction.
U.S. Pat. Nos. 4,889,686; 4,944,904; 4,981,822; 5,015,540; 5,021,367; and 5,043,303, incorporated herein by reference, disclose a molten silicon infiltration formed silicon carbide composite having boron nitride coated reinforcement fibers and the method of formation. Briefly described, an assembly is formed of a fiber reinforced carbonaceous preform and means for contacting the preform with infiltrant, either by placing infiltrant directly on the preform or placing the preform and a deposit of infiltrant on a wicking material such as carbon cloth. The assembly is heated to the infiltration temperature, about 1410.degree. to 1550.degree. C. where the silicon or silicon alloy infiltrant is molten. The infiltrant reacts with the carbonaceous preform to form a composite having a matrix of silicon carbide.
The boron nitride coating helps protect the carbon or silicon carbide reinforcement fibers from reacting with the molten infiltrant. The coated reinforcement fibers can have an outer layer on the inner boron nitride layer, the second layer being a silicon-wettable material such as elemental carbon, metal carbide, a metal which reacts with silicon to form a silicide, metal nitride such as silicon nitride, and metal silicide. The metal carbide can be a carbide of silicon, tantalum, titanium, or tungsten. The metal silicide can be a silicide of chromium, molybdenum, tantalum, titanium, tungsten, and zirconium. The metals which react with silicon are chromium, molybdenum, tantalum, titanium, and tungsten.
Although the protective coatings described above have provided a significant improvement in silicon carbide composites, we have found that such coatings can provide variable performance in improving the strength and toughness of the composites. It is believed variations in coating thickness, coating imperfections such as porosity or pinholes, and consumption of some of the coating by the molten silicon infiltrant are some of the factors that can lead to reaction between the molten silicon infiltrant and localized areas of the reinforcement fibers despite the presence of the coating. For example, batches of several thousand fibers in arrayed stacks can be coated in a chemical vapor deposition coating apparatus, but coating variations along the fiber length or between fibers in the stacked arrays can be found. Reaction between the silicon infiltrant and the fiber, even in localized areas, can result in reduced strength and toughness in the composite, for example by degradation of the fiber strength or the formation of chemical bonds between the reinforcement fibers and the matrix.
One aspect of this invention is to provide molten silicon infiltration formed silicon carbide composites having an improved coating on a reinforcement from a reactive interlayer in the coating.
Another aspect of this invention is to provide a method of forming molten silicon infiltration formed silicon carbide composites having improved protective coatings on the reinforcement phase from a reactive interlayer in the coating.
Another aspect of this invention is to provide an improved protective coating having a reactive interlayer for the reinforcement phase in molten silicon infiltration formed silicon carbide composites.